“All in all, we did not find any effect of birth order on extraversion, emotional stability, agreeableness, conscientiousness, or imagination, a subdimension of openness,” the researchers write.
The evolutionary view of birth order effects is that siblings have to compete for their parents’ attention and favor, and they find different ways to do that. The oldest are physically stronger and mentally more developed than the younger siblings (at least while they’re all still children), so it behooves them to be aggressive (and bossy?). The later-born siblings need more help from others, and so become more extraverted.
None of this bore out in the PNAS study, but logically, if these effects existed, they should be more pronounced when the siblings are young and still living together, since the need to compete for parental attention wanes with age. (Hopefully.) But another study published this summer in the Journal of Research in Personality looked at 377,000 U.S. high-school students, and also found little evidence for personality differences based on birth order.
The researchers looked at the Big Five personality traits—openness, agreeableness, neuroticism, conscientiousness, and extraversion—and predicted that “firstborns (versus laterborns) should be higher in Conscientiousness, Neuroticism, and the dominance aspect of Extraversion, whereas laterborns should be higher in Agreeableness and the sociability aspect of Extraversion.”
Over this entire huge population, they found that firstborns did tend to be a little more conscientious and dominant, and less sociable, but also that they tended to be more agreeable and less neurotic (which goes against the stereotype). But the effect size of this finding was tiny—so tiny that if they hadn’t looked at hundreds of thousands of people, it wouldn’t have been significant at all. When it came to intelligence, firstborns did have an advantage—of one IQ point. As a comparison, another study found that some adolescents’ IQs changed by as much as 20 points within four years.
“Birth order is often invoked as an important variable to explain the development of personality and intelligence within and across families,” the researchers wrote in the high-school birth-order study. “We would have to say that, to the extent that these effect sizes are accurate estimates of the true effect, birth order does not seem to be an important consideration for understanding either the development of personality traits or the development of intelligence in the between-family context.”
Birth order is a bit like a horoscope, in that it presents you with vague enough traits that it’s easy to project them onto yourself. A firstborn might think, “I am a natural leader;” the youngest, “Everyone does love me;” the middle child, “I am a bit of a rebel, aren’t I?” For example, in a graduate thesis written at the University of Wisconsin, Stout, the counseling student Stacey Armitage found that the college students she interviewed generally saw themselves as fitting the Adlerian description of their birth-order role, though there were some qualities that everyone thought they had more than their siblings. Everyone thought they were the most rebellious, for example, and the most considerate.
People do love to sort themselves into categories, though, whether they’re scientifically sound or not. And any idea that’s permeated culture as much as birth order has is bound to have an effect on how people see themselves. Maybe the stereotypes about birth order have more to do with how people compare themselves to their siblings than how they actually are, on their own.